Therapeutic levels of ultrasound have been shown to affect regional blood flow in both skeletal and cardiac muscle. The major objective of this project is to define the mechanisms of action of these effects by studying the hemodynamics of the microvasculature in the cremasteric muscle of rats during ultrasonic exposures. Studies have been conducted during the first and second years of this project which showed that the small arterioles constricted during both ultrasonic and thermal exposures which caused a 2 degrees C rise in local temperature. The rate of vasomotion at the 3A arteriole (30 micron m) level increased as much as 400 percent during the ultrasonic insonation. The major emphasis during the third year of this program will be on the ischemic muscle preparation in both acute and chronic experiments. The hemodynamic parameters to be evaluated include: arteriolar diameters, RBC velocity, flow and systemic pressure.